1. Field of the Invention
The present invention relates to an electrostatic latent image developer and an electrostatic latent image developing method using the same, and more particularly to a novel electrostatic latent image developer applicable to any electrophotographic developing systems such as printers and facsimiles that include development of an electrostatic latent image and also to a developing method using such a developer.
2. Description of the Related Art
Various types of electrophotographic developing systems for developing an electrostatic latent image have conventionally been proposed, and they are generally classified into a one-component developing system and a two-component developing system. The one-component developing system employs a developer containing only a toner for developing an electrostatic latent image formed on a photosensitive member. The one-component developing system has two types, one employing a magnetic toner whereas the other employs a nonmagnetic toner. In both types, a thin developer layer is formed on a support member for supporting the toner. On the other hand, the two-component developing system employs a developer containing 95 to 98 wt % of a carrier and 2 to 5 wt % of a toner mixed together. In this mixing ratio, the toner can uniformly contact the carrier so as to surround it. The toner in this system is a nonmagnetic toner, and if the mixing ratio of the toner is greater than 5 wt %, fog due to the toner occurs in a background portion of a printed image to cause a reduction in image quality.
There has been proposed another system using a magnetic toner as the toner of the developer employed in the two-component developing system to increase the mixing ratio of the toner. This proposed system may be regarded as an intermediate between the one-component developing system and the two-component developing system, and is accordingly called a 1.5-component developing system. The 1.5-component developing system is disclosed in detail in U.S. Pat. No. 4,640,880 (Japanese Patent Publication No. Hei 2-31383), which is incorporated herein by reference. The 1.5-component developing system employs a developer containing 30 to 80 wt % of a carrier and 20 to 70 wt % of a toner mixed together. Also in the 1.5-component developing system, like the two-component developing system, the developer is supported on a developing sleeve incorporating a magnet roll therein to form a developer layer thicker than that in the one-component developing system on the developing sleeve. Accordingly, the 1.5-component developing system has a problem that much fog due to the toner occurs in a background portion of a printed image and additionally the sharpness of lines or letters of the printed image becomes low because of the thick developer layer on the developing sleeve in spite of the low mixing ratio of the carrier.
Further, the bristles of a magnetic brush formed by the developing sleeve and the magnet roll in the 1.5-component developing system are lower in height than those in the two-component developing system. Accordingly, the 1.5-component developing system has an advantage in that a gap between the photosensitive member and the developing sleeve can be made narrower than that in the two-component developing system owing to the reduced height of the bristles of the magnetic brush, thereby effecting faithful development by an electric field generated in the gap between the photosensitive member and the developing sleeve. However, in the 1.5-component developing system, the reproductivity of fine lines is not satisfactory. For example, although the diameter of a dot formed by a laser beam is about 80 .mu.m, the thickness of a line becomes 100 to 150 .mu.m. Accordingly, even when an image having lines spaced a distance corresponding to the dot diameter is printed, the lines printed are joined together, resulting in a defective print similar to a solid image. In addition, such unfaithful reproduction with respect to the dot of the laser beam causes a reduction in clearness of letters due to splash of the toner around the letters.